It can't.
Slavery is, historically, the norm for "civilized" humanity. It's not until you get to a proper industrial revolution that slavery ceases to be economically viable.
Westeros is also a slave based economy; what do you think the smallfolk are?
Hell, even in modern life; wage slave is a term for a reason. Someone working at McDonalds would actually cost the company less than a plantation slave when you adjust for inflation.
I mean iPhones are made with what is effectively slave labor, as are quite a lot of modern conveniences.
So first, define slavery.